Synthesis gas (hereinafter referred to as syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO). Syngas can be produced, in principle, from virtually any material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite; and renewable resources such as lignocellulosic biomass and various carbon-rich waste materials. It is preferable to utilize a renewable resource to produce syngas because of the rising economic, environmental, and social costs associated with fossil resources.
There exist a variety of conversion technologies to turn carbonaceous feedstocks into syngas. Conversion approaches can utilize a combination of one or more steps comprising gasification, pyrolysis, steam reforming, and/or partial oxidation of a carbon-containing feedstock.
Syngas is a platform intermediate in the chemical and biorefining industries and has a vast number of uses. Syngas can be converted into alkanes, olefins, oxygenates, and alcohols. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can also be directly combusted to produce heat and power.
There is commercial interest in producing alcohols from syngas rather than from fermentable sugars. At present, known catalysts for the direct synthesis of higher (C2+) alcohols from syngas produce a mixture of alcohols. For example, the product distribution of methanol, ethanol, propanol, butanol, and higher alcohols often follows a Flory-Schulz distribution. Known catalysts for the synthesis of higher alcohols from syngas also can suffer from low productivities and from declining selectivities to higher alcohols as syngas conversion increases. These factors will tend to require recycle of unconverted syngas to the reactor.
Syngas conversion to methanol, on the other hand, is well-known. For example, syngas (usually derived from natural gas) can be catalytically converted to methanol at very high selectivities using a mixture of copper, zinc oxide, and alumina at a temperature of 250° C. and pressures of 750-1500 psi. In addition to Cu/ZnO/Al2O3, other catalyst systems suitable for methanol synthesis include ZnO/Cr2O3, Cu/ZnO, Cu/ZnO/Cr2O3, Cu/ThO2, Co/S, Mo/S, Co/Mo/S, Ni/S, Ni/Mo/S, and Ni/Co/Mo/S.
Although methanol can be combusted to produce energy, methanol is not currently acceptable as a liquid transportation fuel except in small quantities, e.g. as a minor additive to gasoline. Methanol can, however, be converted to many other fuels and chemicals. With respect to liquid transportation fuels, methanol can be considered a platform intermediate for producing gasoline and biodiesel. It would be useful to convert methanol to specific oxygenates, such as ethanol, for addition to gasoline. Heavier alcohols can also be valuable for chemical applications, as is known.
In light of the aforementioned needs in the art, what are desired are methods, apparatus, and systems that can cost-effectively produce one or more C1-C4 alcohols, such as ethanol and/or 2-propanol, starting with syngas or methanol. It is sought to overcome the poor selectivities associated with alcohol-synthesis catalysts.